Support SMTP connection pooling

What does this MR do?

Adds a new ActionMailer delivery method that supports smtp connection pooling

Testing locally

Add a config/initializers/smtp_settings.rb file with the following contents:

ActionMailer::Base.add_delivery_method :smtp_pool, Gitlab::SMTPPool::DeliveryMethod

Rails.application.config.action_mailer.delivery_method = :smtp_pool
Rails.application.config.action_mailer.perform_deliveries = true

ActionMailer::Base.delivery_method = :smtp_pool
ActionMailer::Base.smtp_pool_settings = {
  ...
  # accepts the same options with :smtp delivery method
}

I also had to set config.eager_load = true in development mode because I was getting autoload deadlocks when multiple notifications were triggered and multiple threads are trying to autoload at the same time.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #230717 (closed)